Can't speak for anyone else, but it is not unusual (nor new) for someone to describe themselves as "Catholic." Briefly, they usually mean that they are a member of the Roman Catholic Church. Wikipedia will provide a great deal of reading about it.

Neither is it unusual for someone to describe themselves as a particular Protestant denomination: Lutheran, Baptist, Methodist, Presbyterian, Episcopalian, et al. Again, Wikipedia is a good starting place.

People who simply describe themselves as "Christian" are what, in my experience at least, is relatively new. Going back, say, fifty years, it was somewhat unusual in many parts of the US to find people who described themselves that way.

In my experience, most of these people belong to one or another of what might be called non-denominational Christian churches. My preferred term for many of them is "contemporary American fundamentalist Christian," but that is not a widely used term, at least not that I know of.

Your question is strange enough that I'm honestly not sure whether or not you're trolling. If you are, as it seems you might be, a member of a contemporary American non-denominational Christian church, it is very weird, whether you know it or not, to suggest that a church that has existed for roughly two thousand years and has many more than a billion members wordwide is "splintering off" and "making [its] own religion."